Frequency modulation (FM) communication receivers are known to comprise a receiver frontend, a discriminator, and a squelch circuit. The receiver frontend includes filters, amplifiers, and downconverters that downconvert a received FM radio frequency signal to an intermediate frequency (IF) signal. The IF signal is typically filtered with an IF filter to remove frequency components outside a desired IF bandwidth. The discriminator demodulates the IF filter's output signal to produce a baseband signal that includes frequency components in a desired audio band and noise.
The squelch circuit receives the demodulated output of the discriminator and detects the presence of an FM modulated radio frequency signal at an input (e.g., an antenna) to the FM receiver. The squelch circuit measures the noise energy of the discriminator's output signal in a frequency band above the desired audio band. For example, if the desired audio band is 0-3 kilohertz (kHz) and the discriminator output signal has frequency components in the 0-40 kHz frequency range, the squelch circuit typically measures noise energy of the discriminator output signal in the 5-20 kHz range. The noise energy in the discriminator output signal varies as a function of the received signal strength and the FM deviation used to modulate the radio frequency signal present at the input to the receiver. The noise energy decreases as the signal strength of the input FM signal increases and increases as the FM deviation increases.
FIG. 3 illustrates typical plots 303,305, 307 of noise magnitude in the receiver's discriminator output signal. Plot 303 exemplifies the noise magnitude when an unmodulated radio frequency signal 301 is received by the receiver. Plot 305 exemplifies the noise magnitude when an FM radio frequency signal 301 is received by the receiver. Plot 307 exemplifies the noise magnitude when no radio frequency signal is present at the receiver's input. As shown in FIG. 3, the noise magnitude, and accordingly noise energy, in the discriminator's output signal is maximum when no radio frequency signal is present and minimum when an unmodulated radio frequency signal 301 is present. As also depicted in the plots 303, 305, 307, the noise energy with a modulated radio frequency signal approaches the noise energy with no radio frequency signal. Further, as the FM deviation of the input FM signal is increased, the noise in the discriminator output also increases and more closely approaches the noise energy with no radio frequency signal present.
The squelch circuit uses the measured noise energy to detect the presence of the FM input signal. The squelch circuit detects the presence of an FM signal when the noise energy in the discriminator output signal decreases below a so-called squelch threshold. The squelch threshold corresponds to the minimum signal energy level at which the squelch circuit will acknowledge the presence of an unmodulated radio frequency signal at the input to the communications receiver. When the squelch circuit detects the presence of the FM radio frequency signal, the squelch circuit sends a control signal to the audio gating portion of the receiver, thereby allowing a user of the receiver to receive the received audio. When the squelch circuit determines that no FM radio frequency signal is present, the squelch circuit sends another control signal to the audio gating portion of the receiver, thereby preventing the user from receiving any audio. However, when the squelch threshold is set to allow reception of FM signals having a particular signal strength and FM deviation, and the receiver receives a signal modulated with a higher deviation, the increase in detected noise energy due to the additional FM deviation often results in an erroneous decision by the squelch circuit (i.e., the squelch circuit determines that no FM signal is present when if fact an FM signal is present). This erroneous decision is known as squelch clamping.
To compensate for changes in discriminator output noise energy due to increased FM deviation, prior art squelch circuits estimate the FM deviation and use the estimated deviation to adjust (e.g., increase) the squelch threshold. The deviation is estimated by measuring the energy in the audio band (i.e., 0-3 kHz) of the discriminator output signal. As is known, the audio band energy is proportional to the FM deviation.
A problem arises with the prior art approaches in modem communication systems, such as those communication systems that comply with the Association of Public Safety Communication Officers Project 25 (APCO 25), which are moving toward narrower IF bandwidths for better spectrum efficiency. Receivers with IF bandwidths approximately equal to twice the transmitted modulation bandwidth inherently produce attenuation of significant sidebands (e.g., first order and second order sidebands) of the IF signal when the modulation frequency is approximately equal to one-half the IF bandwidth. The attenuation of the significant sidebands results in an error in the audio band energy estimate (and equivalently the FM deviation estimate) because the audio band energy, which is contained primarily in the significant sidebands, has been reduced by the narrow IF filter. Thus, the amount of error in the audio band energy estimate is a function of the modulation frequency. For example, an IF filter having a bandwidth of 5.6 kHz would attenuate significant sidebands of the IF signal when the modulation frequency was approximately 2.8 kHz; whereas, the IF filter would not attenuate significant sidebands of the IF signal when the modulation frequency was below 2.8 kHz. However, the prior art approaches do not compensate for the resulting reduction in audio band energy due to the narrow IF filter when the modulation frequency is approximately equal to one-half the IF bandwidth.
Therefore, a need exists for a method and apparatus that determine the audio band energy of a squelch circuit input signal in communication receivers using narrowband IF filters. Further, such a method and apparatus that compensate for the attenuation produced by a narrowband IF filter would be an improvement over the prior art.